Abstract:

NICMOS, the Near Infrared Camera and Multi-Object Spectrometer is a new
instrument for infrared astrophysics on the Hubble Space Telescope.
Due to be installed in the 2nd maintenance mission in early 1997,
it will provide high resolution imaging, coronographic imaging,
spectroscopy and polarimetry capabilities in the 0.8 to 2.5 micron
spectral region.

The scheduled launch date for NICMOS is February 13, 1997, on STS-82, the space
shuttle Discovery. This instrument, now under final integration at Ball
Aerospace under contract to the University of Arizona, will finally fulfill the
initial requirement of infrared observations with HST. NICMOS operates in the
0.8--2.5 micron spectral range utilizing the 256x256 NICMOS 3 HgCdTe detector
arrays developed by Rockwell International and the University of Arizona for
the NICMOS project. NICMOS utilizes three of these detector arrays in three
separate cameras to provide all of the functions of the instrument. A brief
description of their characteristics is as follows.

Camera 1

Pixel size = 0.043 arc seconds, field of view =
1111 arc seconds

Camera 2

Pixel size = 0.075 arc seconds, field of view =
19.219.2 arc seconds

Camera 3

Pixel size = 0.2 arc seconds, field of view =
51.251.2 arc seconds

Camera 1 is diffraction limited at 1.0 microns, Camera 2 at 1.75 microns, and
Camera 3 provides a wide field imaging capability. Each camera has a separate
filter wheel containing a blank for dark images plus 19 filters, grisms, and
polarizers optimized to the camera characteristics. Camera 2 provides a true
coronographic capability with a 0.63 arc second diameter occulting spot and a
cold apodizing mask at the cold pupil. Camera 3 contains three grisms for
multi-object spectroscopy and there are a total of six polarizers in Cameras 1
and 2. The three polarizers in Camera 1 are centered at 1 micron and have 120
degree angular rotations relative to each other. The polarizers in Camera 2
are centered at 2 microns again spaced in rotation by 120 degrees.

Figure 1 indicates the NICMOS field of view and camera placement
relative to the HST pickles and the current instruments. NICMOS will replace
the FOS while STIS will be placed in the current GHRS position.

Figure: The NICMOS field of view.

The key to the imaging program is the NICMOS 3 detector characteristics. The
measured flight detectors have rather similar characteristics. All NICMOS 3
detectors have 4040 micron pixel areas in a 256256 format. The
pixel to pixel crosstalk is less than 0.2% and the pixels are individually
read out which eliminates row or column smearing due to charge transfer. The
average read noise is 35 electrons and the average dark current is 0.1
electrons per second at the operating temperature of 58 K. The quantum
efficiency varies from a high of about 80% at 2.4 microns to a low of about
15% at 0.8 microns. The detector readouts are non-destructive which means
that the detector may be read out at any time without destroying the image.
This feature offers attractive alternatives such as examining the image at
various time during the integration to eliminate cosmic ray effects or multiple
reads at the beginning and end of the integrations to reduce the read noise.

Figure 2 gives an indication of the sensitivity of the NICMOS
instrument. We have picked the F160W filter in Camera 3 which operates near
the minimum of the background in NICMOS and roughly is equivalent to the H
filter in ground-based imaging. The figure contains four panels. The first
shows the transmission of the filter. The second panel indicates the time
required to reach a signal to noise of 1, 3, and 10 for a point source of
strength given in either Janskys or H Magnitudes. In this calculation only the
central pixel is utilized, so some gain is expected through the proper weighted
summing of other pixels. The third panel is the same calculation only using
the flux in a line if it is the only source of illumination of the pixel.
Finally the last panel indicates how long a source of a given strength can be
integrated before non-linear effects start to appear. This limit is on the
order of 200,000 electrons.

Figure: The NICMOS Sensitivity

One of the advantages of the NICMOS cameras is their ability to operate
independently of each other. For example, during a long exposure in one camera
the other cameras can take a series of short exposures in a set of other
filters. This can be useful when one camera is perhaps on the nucleus of a
galaxy and the others are observing parts of the outer regions.

Grisms in Camera 3 provide the NICMOS spectroscopic capability. The grisms
operate in a slitless mode to provide spectra of all of the objects in the
field. The absence of strong telluric OH emission makes this mode attractive
for NICMOS. In crowded fields, spectra will be required at more than one
orientation angle to eliminate cross talk between spectra.

There are three grisms in the filter wheel of Camera 3, centered on 0.964,
1.410, and 2.058 microns with average resolutions per pixel of 200. The first
two grisms are quite sensitive, however, the long wavelength grism suffers from
the thermal emission from HST. It is provided mainly to ensure complete
spectroscopic coverage of the NICMOS spectral region. Figure 3
shows the grism performance in the same manner as the imaging performance in
the Figure 2. This particular case looks at the performance at the
[Fe II] line at 1.644 microns.

NICMOS is the first instrument to contain a true coronograph in HST with the
proper correct optics to utilize it. Camera 2 is the coronographic camera in
that it has a 0.63 arc second diameter occulting spot in the middle of one
quadrant of the field. This spot is a hole in the imaging mirror so it is a
permanent part of the camera field. The second part of a coronographic system
is an apodizing stop at a pupil. All of the NICMOS cameras have a cold pupil
stop inside of the dewar with apodizing masks. These masks are required for
thermal suppression in addition to their coronographic properties. The mask for
Camera 1 masks the secondary hole, primary edge, and spiders. The masks for
Cameras 1 and 3 also mask the hold-down pads on the primary. Camera 2, however,
is the only camera with an occulting spot. Use of the coronographic mode will
require special acquisition techniques to place the central object on the
occulting spot.

NICMOS provides short wavelength polarization capability centered on 1 micron
in Camera 1 and medium wavelength polarization centered on 2 microns in
Camera 2. Each of the two cameras contains three polarizers separated in angle by
120 degrees. With these polarizers, the direction and strength of linear
polarization at any angle may be obtained without rolling the telescope. Since
there are angular reflections in NICMOS there is an instrumental polarization
of about 5%. The polarization capabilities of NICMOS are oriented toward
regions of high polarization such as is found in many star formation regions.

There are four basic modes of operating the NICMOS cameras. Each mode is
designed for a specific purpose such as noise reduction or cosmic ray
detection. In addition, there are subsets of these modes which are currently
being implemented by the team and STScI which will be detailed in later reports
on the instrument. The basic modes are as follows:

ACCUM

N non-destructive reads at the start and N non-destructive
reads at the end of the integration, where N is a user-specified number.
The image is the difference between the average of the initial and the
average of the final reads.

MULTI-ACCUM

A non-destructive read at the beginning, end, and at
user-specified times during the integration. All readouts are returned to the
observer. This mode monitors the progress of the integration during the
observation.

RAMP

N evenly spaced non-destructive reads during the integration
fit by a linear function. This mode includes options of cosmic ray correction
and the detection of saturation. N is specified by the proposer.

BRIGHT OBJECT

Pixel by pixel integration for sources that would
saturate the detector in the normal cycle time through the whole array.

The following offers some short tips on expected observing strategy with
NICMOS. It is very probable that an orbit experience will alter some of these
strategies.

For low read noise:

Use the ACCUM mode with N at least 10 or employ
the RAMP mode with N at least 20.

For good cosmic ray rejection:

Perform at least three independent
observations in the ACCUM mode, use MULTI-ACCUM with a least three observations
or use the RAMP mode with cosmic ray rejection turned on.

To reach the natural background limit:

Integrate long enough to be on
the square-root slope of the source strength versus time curve. This time
will vary greatly among the filters. For short wavelength narrow band filters,
the detector noise will be the limit. For objects with source fluxes greater
than the background, you will be source-noise limited rather than background
limited.

For background subtraction:

In some cases it will be prudent to arrange
your observing to provide for background subtraction. At the longer
wavelengths, this will be generally a requirement. For point sources take at
least nine independent integrations spaced by small increments of the prime camera
field of view. The background is then determined by median filtering. Use the
spacecraft small angle maneuvers unless you are in parallel with another
instrument. Note the built-in patterns provided in the proposal instructions.
For extended sources, take at least nine pairs of observation with the source in
and out of the field of view. Again, the built-in patterns are very useful.
Note that small angle maneuvers or field offset mirror motions affect all of
the cameras. It is generally not possible to have effective background
subtraction in all cameras simultaneously. Also note that the long wavelength
filter and grism observations are strongly affected by the thermal background
and will require background subtraction. Use them only if the science requires
it.

Grism spectroscopy:

Take a continuum image in the filter closest to the
grism band before and after the grism observation to provide the base point for
the spectrum. If possible, take observations with different roll angles to
identify and remove overlapping spectra. Also note that since the grisms are
slitless, extended objects will degrade the spectral resolution.

Coronographic imaging:

Coronographic imaging requires an acquisition
procedure that places the central bright object behind the occulting spot.
Also note that it is very difficult to do background subtraction in the
coronographic mode.

Acknowledgments:

This work is a result of work by the entire NICMOS Instrument Definition Team.
NICMOS is being built by the University of Arizona under contract from the
National Aeronautics and Space Administration.